NAME¶

std::type_identity - std::type_identity

Synopsis¶

Defined in header <type_traits>
template< class T > (since C++20)
struct type_identity;

Provides the member typedef type that names T (i.e., the identity transformation).

If the program adds specializations for std::type_identity, the behavior is
undefined.

Member types¶

Name Definition
type T

Helper types¶

template< class T > (since C++20)
using type_identity_t = type_identity<T>::type;

Possible implementation¶

template<class T>
struct type_identity { using type = T; };

Notes¶

std::type_identity can be used to establish non-deduced contexts in template
argument deduction.

Feature-test macro Value Std Feature
__cpp_lib_type_identity 201806L (C++20) std::type_identity

Example¶

// Run this code

#include <iostream>
#include <type_traits>

template<class T>
T foo(T a, T b) { return a + b; }

template<class T>
T bar(T a, std::type_identity_t<T> b) { return a + b; }

int main()
{
// foo(4.2, 1); // error, deduced conflicting types for 'T'
std::cout << bar(4.2, 1) << '\n'; // OK, calls bar<double>
}

Output:¶

5.2

See also¶

identity function object that returns its argument unchanged
(C++20) (class)